1. Field of the Invention
The present invention relates to a sheet feeding device having a sheet loosening mechanism that employs warm air assistance, and an image forming apparatus including the sheet feeding device.
2. Description of the Related Art
In a conventional image forming apparatus such as a printer, a copier, or a facsimile, cut sheets of high quality paper, regular paper specified by a copier manufacturer, and so on are typically used as a sheet fed continuously into an image formation portion. A cut sheet of high quality paper, regular paper, and so on has low surface smoothness, and therefore a sheet sticking force thereof is comparatively low. It is therefore comparatively easy to prevent multi-feeding, in which a plurality of cut sheets are supplied while stuck together, when dispatching the cut sheets one at a time from a sheet carrying unit such as a sheet feeding tray. Furthermore, even when multi-feeding occurs during use of the cut sheets, the cut sheets can be dispatched one at a time comparatively favorably by providing a separating roller, a separating pad, a separating pawl, or similar.
In recent years, however, diversification of sheet has progressed to the point where not only sheets of high quality paper, regular paper, and so on having low surface smoothness are used. In particular, as colorization techniques become more advanced in image forming apparatuses, the use of recording media having high surface smoothness, such as enhanced-whiteness gloss enamel paper (composite paper coated on one or both sides with a coating color, which is a type of paint, with the aim of improving printing suitability), is becoming more widespread. In other words, not only high quality paper and regular paper, but also the enamel paper described above as well as film sheets, tracing paper, and so on are used in the same machine type. Enamel paper, film sheets, tracing paper, and so on exhibit a strong inter-sheet sticking force, and it is therefore difficult to prevent multi-feeding of the sheets. Hence, special measures must be taken in relation to sheet feeding (sheet dispatch).
Furthermore, an upper face and a peripheral part of a stack of sheets disposed on the sheet carrying unit are exposed to outside air, and are therefore likely to contain a large amount of moisture. In other words, the upper face and side faces of the sheet stack swell due to moisture absorption, whereas the degree of swelling on the inside of the sheet stack is lower than that of the upper face and side faces due to the smaller amount of moisture. As a result, pressure inside (in the inter-sheet spaces of) the sheet stack may turn negative such that the sheets stick together.
To loosen the sheet stack by separating sheets that are stuck together prior to sheet feeding, large copiers and so on employ a sheet feeding device having a mechanism (to be referred to hereafter as “lateral warm air assistance”) for blowing warm air onto the side face of the sheet stack. For example, Japanese Patent Application Laid-open No. 2001-48366 discloses a technique for improving sheet drying efficiency in a sheet loosening method employing lateral warm air assistance by appropriately adjusting the humidity of lateral warm air that is blown onto the side face of a sheet stack.
However, in the conventional sheet loosening technique employing lateral warm air assistance described above, it is difficult for the warm air to reach regions remote from a warm air blowing port, and it is therefore difficult to loosen the sheets by introducing warm air into the vicinity of the outer periphery of the sheets, in which the sticking force is particularly strong. In other words, when lateral warm air assistance is used conventionally, required warm air blowing means, heating means, a power supply, and so on must all be large to obtain a favorable loosening effect. Therefore, conventional sheet loosening techniques employing lateral warm air assistance are limited to application to comparatively large sheet feeding decks accommodating between approximately 2000 and 4000 sheets.